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Recent Status and Trends of the Land Bird Avifauna on Saipan, Mariana Islands, with Emphasis on the Endangered Nightingale Reed-Warbler Acrocephalus Luscinia

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Recent Status and Trends of the Land Bird Avifauna on Saipan, Mariana Islands, with Emphasis on the Endangered Nightingale Reed-Warbler Acrocephalus Luscinia Bird Conservation International (2009) 19:323–337. ª BirdLife International, 2009 doi:10.1017/S0959270909008417 Recent status and trends of the land bird avifauna on Saipan, Mariana Islands, with emphasis on the endangered Nightingale Reed-warbler Acrocephalus luscinia RICHARD J. CAMP, THANE K. PRATT, ANN P. MARSHALL, FRED AMIDON and LAURA L. WILLIAMS Summary The avifauna of the Mariana Islands, an archipelago in the western Pacific, faces the threats of rapid economic development and the spread of non-native species, particularly a devastating predator, Brown Tree Snake Boiga irregularis. In this paper, we examine the status and trends of the land bird fauna of Saipan Island based on three island-wide surveys conducted in 1982, 1997, and 2007. During this period, the human population on Saipan increased more than four-fold and much of the island has been developed. The surveys employed standard point-transect methods based on Distance Sampling. Remarkably, we found nearly all species of land birds - 11 native species and three introduced species - to be common or abundant. The exception was the Micronesian Megapode Megapodius laperouse, a historically rare species that was not observed on the 2007 survey, although it does persist on Saipan and other Mariana islands. A comparison of species densities among the three surveys showed that seven species, mainly fruit and seed- eaters, had increased and three species of insectivorous birds had decreased - Rufous Fantail Rhipidura rufifrons, Nightingale Reed-warbler Acrocephalus luscinia, and Golden White-eye Cleptornis marchei. Of these three, Nightingale Reed-warbler is listed as ‘Endangered’ on the IUCN Red List and as an Endangered Species by the U.S. Fish and Wildlife Service. Reed-warbler densities on Saipan decreased by more than half between 1982 and 2007. Although point transect sampling worked well for this species, density estimates and trends assessment could be im- proved by reallocating sampling stations among habitats and by more frequent sampling. Introduction The Mariana Islands, an archipelago of 15 islands east of the Philippines, is one of the Endemic Bird Areas of the Pacific identified by BirdLife International (Stattersfield et al. 1998). The native land bird avifauna is composed of 19 species, of which 16 are Restricted Range species (Stattersfield et al. 1998) and 10 are endemic to the archipelago. Most of the endemic birds are closely related to species on other nearby Pacific islands, except for the Golden White-eye which is a monotypic genus. Over the past several centuries, the Mariana avifauna has withstood environmental changes brought about by ever-increasing human settlement of the islands. Matters changed after World War II with the introduction of the Brown Tree Snake to Guam, where it eliminated most bird species and decimated the rest (13 of 22 native breeding birds were lost from Guam; Rodda and Savidge 2007). Hope for the remaining Mariana avifauna now lies with the islands north of Guam - Rota, Aguiguan, Tinian, and Saipan - the main islands of the Commonwealth of the Northern Mariana Richard J. Camp et al. 324 Islands (CNMI; Figure 1). Although there have been recent reports of the Brown Tree Snake from some CNMI islands, particularly from the most populous island, Saipan, surveys to date have not demonstrated that this notorious bird predator has established a breeding population in the CNMI (Rodda and Savidge 2007). Nevertheless, it would seem that without intensive interdiction Figure 1. Island of Saipan showing the survey transects. Status and trends of Saipan birds 325 efforts, the snake will soon establish itself on Saipan, if it has not already done so (Colvin et al. 2005). Other invasive non-native species are also a concern. Scarlet Gourd Coccinia grandis is an invasive herbaceous vine that was probably introduced to Saipan in 1998, where it has altered bird habitat by smothering native and non-native forest, Tangantangan Leucaena leucocephala thicket, and other vegetation. These newly emerging alien species have arrived with the exchange of goods and trade as part of ongoing globalisation (Heuttmann and Czech 2006). Apart from the Brown Tree Snake and other non-native species, the main threat to Saipan’s native birds has been extensive economic development of the island with resulting trans- formation of forested and agricultural habitats to residential housing, resorts, golf courses, and other commercial building. Invasive species and economic development have been the cause for extinction or depletion of numerous bird species in Oceania and would seem to bode poorly for birds in the CNMI as well (BirdLife International 2000). Many of the 12 species of native land birds on Saipan are believed to be common and doing fairly well (Table 1; Reichel and Glass 1991). Nevertheless, five species appear on the IUCN Red List (BirdLife International 2000, updated at www.iucnredlist.org and viewed 29 December 2008). The Golden White-eye is designated as ‘Critically Endangered’. Table 1. List of bird species and numbers of birds detected from point-transect surveys in three different years, Saipan, Mariana Islands. Nomenclature for widespread species follows the American Ornithologists’ Union (1983) and for restricted range species, BirdLife International (2000, updated at www.iucnredlist.org). Density estimates were produced for species in bold. Class includes: native land bird (N), introduced land bird (I), domestic (D), seabird (S), vagrant (V), and water bird (W). Species Name Scientific Name Class 1982 1996 2007 White-tailed Tropicbird Phaethon lepturus S 18 5 3 Yellow Bittern Ixobrychus sinensis N 14 40 30 Pacific Reef Heron Egretta sacra W 010 Lesser Cattle Egret Bubulcus ibis W 021 Micronesian Megapode Megapodius laperouse N 510 Red Junglefowl Gallus gallus D 327 2 47 Common Peafowl Pavo cristatus D 600 Wild Turkey Meleagris gallopavo D 501 Northern Bobwhite Colinus virginianus D 001 Common Moorhen Gallinula chloropus W 100 Pacific Golden Plover Pluvialis fulva W 601 Black Kite Milvus migrans V 010 Brown Noddy Anous stolidus S 03129 Black Noddy Anous minutus S 0012 Common White-tern Gygis alba S 574 257 395 Rock Dove Columba livia D 0102 Island Collared-dove Streptopelia bitorquata I 29 74 89 White-throated Ground-dove Gallicolumba xanthonura N 11 24 172 Mariana Fruit-dove Ptilinopus roseicapilla N 487 319 367 Guam Swiftlet Collocalia bartschi N 106 153 266 Collared Kingfisher Todiramphus chloris N 339 345 297 Micronesian Myzomela Myzomela rubratra N 549 317 474 Rufous Fantail Rhipidura rufifrons N 1128 691 474 Nightingale Reed-warbler Acrocephalus luscinia N 287 190 118 Bridled White-eye Zosterops saypani N 1864 1702 1750 Golden White-eye Cleptornis marchei N 749 426 373 Micronesian Starling Aplonis opaca N 98 169 253 Eurasian Tree Sparrow Passer montanus I 41 371 324 Orange-cheeked Waxbill Estrilda melpoda I 0075 Richard J. Camp et al. 326 The Micronesian Megapode, Mariana Fruit-dove Ptilinopus roseicapilla, and Nightingale Reed- warbler are listed as ‘Endangered’, and the White-throated Ground-dove Gallicolumba xanthonura is ‘Near Threatened’. The U.S. Fish and Wildlife Service (2004) lists the megapode, reed-warbler and Guam Swiftlet Collocalia bartschi as Endangered Species. The aim of this paper is to examine the status and trends of native Saipan land birds over the past 25 years based on a comparison of three surveys. A baseline survey of Saipan was undertaken in 1982 by Engbring et al. (1986), and two follow-up surveys that have not yet been reported were conducted in 1997 (U.S. Fish and Wildlife Service 1998a) and 2007. We analysed data from these surveys to produce estimates of population density and to assess species trends. We also make recommendations for the design of future surveys on Saipan. A particular focus of this study was to determine the status of the ‘Endangered’ Nightingale Reed-warbler. This species is known from six islands in the Mariana archipelago, but it currently persists on only two islands - Saipan and Alamagan (Reichel et al. 1992, U.S. Fish and Wildlife Service 1998b, Berger et al. 2005). On Saipan, the reed-warbler population is limited by habitat loss and predation, especially from introduced rat species (Mosher 2006). The need to maintain habitat for this bird is conflicting with development pressures. The U.S. Fish and Wildlife Service and CNMI Division of Fish and Wildlife are formulating an island-wide conservation plan for the reed-warbler on Saipan, and an important first step is to determine the species’ current status. Methods Survey area 2 Saipan is the largest of the CNMI islands at 119 km (15o 129 N, 145o 459 E). The island consists of low-lying plateaus and a rugged limestone ridge dominated by Mt. Takpochao (436 m). The vegetation of Saipan is mixed second-growth native forest, grassy savanna, and forest of in- troduced trees, mostly Tangantangan thickets (Falanruw et al. 1989). The little native vegetation that remains on Saipan has been greatly altered by centuries of human use and invasion of non- native species (Engbring et al. 1986). An appreciation of the magnitude of environmental transformation of Saipan can be gained from the human population figures for this small island. Records show that the population rose by 429% for the 20-year period from 1980 to 2000 (the most recent survey), by which time it had reached 62,393 people and was still growing (U.S. Bureau of the Census 1981, U. S. Census Bureau 2001). Bird surveys The baseline survey in 1982 sampled a total of 244 stations on 14 transects with representative island-wide coverage across geography and habitats, except that older urban areas were not included (Engbring et al. 1986). Effects of excluding urban areas may result in under-estimating abundances of human commensal species, such as Island Collared-dove Streptopelia bitorquata and Eurasian Tree Sparrow Passer montanus. Transects were originally located using a random- systematic method when applicable (see Engbring et al.
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